This invention relates generally to horizontally burning high intensity discharge (“HID”) lamps with a vaporizable lamp fill material, and to an arc tube therefor.
HID lamps with a vaporizable lamp fill material such as metal halide and ultra high performance mercury lamps have been developed as a point source. Many HID lamps with a vaporizable fill material have found widespread use in outdoor and indoor applications. In many applications, metal halide lamps have been favored because of their higher efficiency in producing white light. However, most arc discharge lamps for general lighting applications are universal, i.e., they are designed to operate in any orientation. When the burning position of a conventional arc discharge lamp is changed from the vertical to the horizontal position, a dramatic shift in light output and color temperature generally occurs.
For example, metal halide lamps designed to operate only in a horizontal position are well known. When a metal halide lamp is operated in the horizontal position, the arc discharge is not coaxial with the horizontally disposed electrodes but tends to bow upwards because of convection currents within the arc tube. The bowing of the arc heats up the upper part of the arc tube and creates a cold spot along the bottom where the halides condense. The resulting large difference in temperature between the upper and lower parts of the arc tube creates performance problems. Depending on the temperature of the cold spot, the halides may condense in such a manner that the surface area of the halide pool is significantly reduced and the vapor pressure of the halides in the arc tube is decreased correspondingly, degrading the light output and the quality of color. In addition, the upper portion of the arc tube may overheat resulting in possible devetrification and increased halide reaction with silica. Thus, the life and efficiency of the lamp are compromised.
The prior art has repeatedly tried to obviate the difficulties which result from the bow in the arc and the resultant difference in temperature between the upper and lower portions of the arc tube. For example, the Koury U.S. Pat. No. 3,858,078 dated Dec. 1, 1974 and the Karlotski U.S. Pat. No. 4,498,027 dated Feb. 5, 1985 disclose the use of an arched arc tube so that the arched shape of the arc tube conforms generally to the bow in the arc during normal operation. However, such arched arc tubes generally have a low spot behind the electrodes and away from the arc, thus creating an undesirable cold spot where the halides condense into a pool having a relatively small surface area.
It is know to locate the electrodes slightly below the curved axis of such arched arc tubes in an effort to heat the end cavity of the arc tube as disclosed., e.g., in the Gungle U.S. Pat. No. 4,056,751 dated Nov. 1, 1977. This adds complexity in the manufacture of the arc tubes but has not obviated the problem of halide condensation behind the electrodes and increased the cost of manufacturing.
Others have attempted to increase the distance from the arc to the top of the arc tube and to increase the temperature of the lower portion of the arc tube by lowering the electrodes relative to the axis of the arc tube without arching the arc tube. As shown in FIGS. 1, 2 and 6, a cylindrical arc tube body 10 may be pinched on the ends 12, 14 and the axis of the electrodes 20 lowered below the axis 24 of the arc tube body. As illustrated in FIG. 3, the cross-sectional shape of the upper portion of the arc tube between the free ends of the electrodes may be modified to increase the distance from the top wall from the arc relative to the side walls. The Rigden U.S. Pat. No. 4,232,243 dated November, 1980, the Howles U.S. Pat. No. 4,001,623 dated Jan. 4, 1977, and the Kowalczyk U.S. Pat. No. 5,525,863 dated Jun. 11, 1996 are exemplary of such efforts.
However, this approach has not solved the problem of cold spots below and behind the electrodes where the halides will condense, and the location of the electrodes below the longitudinal axis of a generally symmetrical arc tube will create wide and unacceptable variations in color quality as the size and location of the surface area of the halide pool changes when the burning position is tilted slightly from the horizontal in the installation of the lamp into a fixture.
More recently, a successful approach is described in the Sulcs U.S. Pat. No. 5,055,740 dated October 8, 1991 assigned to the assignee of the present invention. In this approach, and as shown in FIG. 4, the electrodes 20 are located below the central axis 24 of a cylindrical arc tube which is asymmetrically pinched at 12, 14 to shape the tube at both ends and to move the halide pool toward the central portion thereof. This solves the problem of the overheating of the upper portion of the arc tube and the variation in color with the tilting of the lamp because only the upper portion of the arc tube is arched by the asymmetrical pinch. However, even here, the amount of surface area of the halide condensate is less than desirable and the arc tube wall below and around the electrodes tends to overheat because of proximity to the electrodes.
Attempts have been made to flatten the bottom portion of a cylindrical arc tube to move it closer to the arc and thereby reduce the temperature differential between the top and bottom portions of the arc tube. Such an arc tube is disclosed in the Gordin et al U.S. Pat. No. 5,0016,150 dated May 14, 1991. However, and as explained in greater detail in the aforementioned Kowalczyk U.S. Pat. No. 5,525,863 and as illustrated in FIG. 5, the flattening of the lower wall does not address the overheating of the upper wall and results in longitudinal zones of irregular curvature where the distance RA from the axis of the electrodes 20 is greater than the distance R from the electrode axis to the top and side walls of the arc tube.
Still others have attempted to address the cold spot problem by coating the ends of arc tubes with an infrared reflective coating to raise the temperature of the cold spots behind the electrodes. However, such coatings do not address the cold spot problem at the bottom of a horizontal burning arc tube.
It is accordingly an object of the present invention to obviate many of the problems associated with horizontal burning arc discharge lamps and to provide a novel horizontal burning HID lamp, arc tube and method of arc tube construction.
It is another object of the present invention to provide a novel horizontally burning arc discharge lamp and method with more uniform temperature distribution over the wall of the arc tube.
It is still another object of the present invention to provide a novel horizontally burning arc discharge lamp and method with an increased condensate surface area.
It is yet another object of the present invention to provide a novel horizontally burning arc tube and method in which the electrodes are lowered to accommodate for the bow in the arc and the lower portion of the arc tube is flattened and thus brought closer to the arc, resulting in a lower difference in temperature between the upper and lower portions of the arc tube and an increased condensate surface area in the pool.
It is still yet another object of the present invention to provide a novel horizontally burning arc tube and method in which the electrodes are lowered to accommodate for the bow in the arc and the sides of the arc tube are brought closer to the arc, resulting in a more uniform arc tube wall temperature.
Another object of the present invention is to provide a novel horizontally burning metal halide arc tube and method with improved light output and color quality, and less
Yet another object of the present invention is to provide a novel horizontally burning arc tube and method of increasing the temperature of the lamp at which condensation occurs.
Still other objects of the present invention are to provide novel methods of improving the performance of horizontally burning arc tubes and lamps.
Still further objects of the present invention are to provide novel horizontal burning arc tubes and novel methods of constructing them.
These and many other objects and advantages of the present invention will be apparent to one having skill in this art from the following detailed description of preferred embodiments when read in conjunction with the accompanying drawings.